1. Field of the Invention
The present invention relates to a rotary table unit adapted to be used as a rotatable work table of machine tools, measuring machines, and so on. More particularly, the present invention relates to an accurate rotary table unit adapted for use in the field of ultra precision machining of workpieces, such as microminiature components.
2. Description of the Related Art
Generally, a rotary table unit is constructed so that a table element rotatably mounted on a base element via guide means is driven by a drive source, such as an electric motor. In conventional high precision rotary table units, the table element is slidably supported on a guide portion of the base element, and lubricant oil is filled between the guide portion of the base element and the table element. The table element is rotated by a single drive motor which is connected to the shaft of the table element.
However, with such a sliding guide, if a local load, such as a grinding load, is applied to the table element in a direction perpendicular to the rotating direction of the table element, the table element is shifted due to the change in the gap in which the lubricant oil is filled. As a result, the workpiece on the rotary shifts relative to a machining element, such as a cutting tool. Accordingly, dimensional error of the workpiece occurs.
Such a local load also causes stick-slip motion of the table element on the slide guide means, which results in uneven rotation of the table element and reduced accuracy. In addition, the use of only a single drive motor is apt to result in irregular rotation of the table element or a slow down in the rotating speed of the table element when a local load is applied. Again, this results in dimensional error of the workpiece.
The above-described problems of conventional rotary table units mean lower reliability in use in the field of ultra precision machining where a high accuracy of machining on an order of 0.1 micrometer is needed, and aggravate the problems to be solved for such machining. For example, when a conventional rotary table unit is used for precision machining of a silicon wafer to obtain a mirror finish, the machined surface of the silicon wafer often suffers from an uneven finish or a striped pattern.